1. Field of the Invention
The present invention relates to an active matrix LED display driving circuit and particularly to an organic light emitting diode (OLED) display driving circuit having a simple circuit structure, small circuit area and low power consumption as well as providing a high contrast ratio.
2. Description of the Prior Art
FIG. 1 is a diagram showing a conventional active matrix OLED driving circuit. In each pixel, there are four N-type transistors 11˜14, an OLED 15 and a capacitor 16. The transistor 11 has a drain coupled to receive a data signal IData, and a gate coupled to receive a scan signal Vselect. The transistor 12 has a drain coupled to receive the data signal IData, and a gate coupled to receive the scan signal Vselect. The transistor 13 has a drain coupled to the source of the transistor 12, and a gate coupled to the source of the transistor 11. The transistor 14 has a drain and gate commonly coupled to receive a power supply voltage VDD, and a source coupled to the source of the transistor 12. The OLED 15 has an anode coupled to the source of the transistor 13 and a cathode coupled to the ground. The capacitor 16 is coupled between the drain of the transistor 14 and the gate of the transistor 13. Since all the transistors 11˜14 are N-type transistors, they can be amorphous Si thin-film transistors (a-Si TFTs).
The capacitor 16 is mainly used for charge storage. During a scan period, the transistors 11 and 12 are turned on by the scan signal Vselect so that the data signal IData drives a current through the transistor 13 and charging the capacitor 16. At the end of the scan period, the transistors 11 and 12 are turned off by the scan signal Vselect so that the current driven by the data signal IData is cut off. The voltage established by the charges on the capacitor 16 succeeds the data signal IData to drive the same current through the transistor 13 until the beginning of the next scan period.
The previously described driving circuit has a relatively narrow range of the current through the transistor 13. If a larger data signal IData is used in order to raise the brightness of the OLED 15, the gate-to-source voltage of the transistor 14 will be increased. The drain-to-source voltage of the transistor 13 will decrease as the transistor 14 increases. Accordingly, the transistor 13 will operate in the linear region rather than saturation region if the data signal IData is large enough. This adversely pulls down the current through the transistor 13 to drive the OLED 15. If a higher voltage VDD is used for a higher brightness, the transistor 14 in each dark pixel will be mistakenly turned on beyond the scan period since the dark current through the transistor 13 will be too small to maintain a high enough voltage level on the drain of the transistor 13. Therefore, the range of the variation of the current driving the OLED 15 is limited, which lowers the contrast ratio of the display.
FIG. 2 is a diagram showing another conventional active matrix OLED driving circuit. In each pixel, there are four N-type transistors 21˜24, an OLED 25 and a capacitor 26. The transistor 21 has a drain coupled to receive a data signal IData, and a gate coupled to receive a scan signal Vselect. The transistor 22 has a drain coupled to receive the data signal IData, and a gate coupled to receive the scan signal Vselect. The transistor 23 has a drain coupled to the source of the transistor 22, and a gate coupled to the source of the transistor 21. The transistor 24 has a drain coupled to receive a power supply voltage VDD, a gate coupled to a control signal Vctrl, and a source coupled to the source of the transistor 22. The OLED 25 has an anode coupled to the source of the transistor 23 and a cathode coupled to the ground. The capacitor 26 is coupled between the drain of the transistor 24 and the gate of the transistor 23. Since all the transistors 21˜24 are N-type transistors, they can be a-Si TFTs.
In the circuit of FIG. 2, the problem in the circuit of FIG. 1 is solved by providing the external control signal Vctrl to the transistor 24 so that the variation range of the driving current is wider. However, this requires additional wiring and circuits for the signal Vctrl.
FIG. 3 is a diagram showing still another conventional active matrix OLED driving circuit. In each pixel, there are six N-type transistors 31˜34, 37, and 38, an OLED 35, and a capacitor 36. The transistor 31 has a drain coupled to receive a data signal IData, and a gate coupled to receive a scan signal Vselect. The transistor 32 has a drain coupled to receive the data signal IData, and a gate coupled to receive the scan signal Vselect. The transistor 33 has a drain coupled to the source of the transistor 32, and a gate coupled to the source of the transistor 31. The transistor 34 has a drain coupled to receive a power supply voltage VDD and a source coupled to the source of the transistor 32. The OLED 35 has an anode coupled to the source of the transistor 33 and a cathode coupled to the ground. The capacitor 36 is coupled between the drain of the transistor 34 and the gate of the transistor 33. The transistor 37 has a drain and gate commonly coupled to receive the power supply voltage VDD, and a source coupled to the gate of the transistor 34. The transistor 38 has a drain coupled to the source of the transistor 37, and a gate coupled to receive the scan signal Vselect and a source coupled to the ground. The transistors 37 and 38 act as an inverter. Since all the transistors are N-type transistors, they can be a-Si TFTs.
In the circuit of FIG. 3, there are additional transistors used as an inverter to consume more power and have a large circuit area.